1. Field of the Invention The present invention relates to injection moulding of plastics material.
2. Description of Related Art
It is known that large, thin articles are difficult to form by injection moulding. The reason is that the gap between the two parts of the mould is small and the distance that the material has to travel is too long for the pressure applied by the moulding machine to be available at the far end of the gap from the injection point for driving the plastics to fill the mould. In short, the “flow path thickness ratio” is too long.
Conventionally, thin articles are formed by vacuum or pressure forming where a sheet of plastics material is stretched to conform to the shape of a mould. Such techniques are limited in their application as they cannot produce articles of even wall thickness or articles that have regions of increased or reduced wall thickness. This is because only one surface of the article is being moulded and the thickness at any point is determined exclusively by the thickness of the original sheet and the extent of its deformation.
The present invention seeks therefore to provide a method suitable for moulding an article of thin wall section in which all the surfaces of the article are defined by the wall surfaces of a mould cavity.
In its broadest aspect, the invention a method of moulding a plastics material in a mould cavity which relies primarily on movement of a part of the mould to provide the pressure necessary to force the plastics material melt to fill all the parts of the mould cavity, the method comprising the steps of:
applying a light pressure to close the mould,
injecting a predetermined quantity of molten plastics material into the mould cavity at a pressure which is such that the injection of the plastics material can cause the cavity to expand in volume against the resistance of the light closing pressure, and
applying a high pressure to close the mould fully after completion of the injection step.
The invention may broadly be regarded as applying to plastics material a technique similar to that used in metal forging. A quantity of molten plastics material is placed in the mould cavity while it is not at its minimum volume and plastics material is then compressed by fully closing the mould to force the plastics material into all parts of the mould cavity.
It is known to move part of a mould in order to apply additional compression after having injected a plastics melt into a mould cavity in the conventional manner. This process, which is known as injection compression moulding (ICM) offers advantages of longer flow lengths, thinner walls and a lower level of material stresses. This makes the process suitable for moulding such articles as CD's and DVD's (because of improved internal stresses) and vehicle body and instrument panels (because of improved impact resistance).
The known ICM process differs from the present invention in that the plastics melt is introduced into the mould under substantial pressure and during the injection of the plastics melt the mould parts are held together with sufficient force for the mould cavity to remain of constant volume during the injection. By contrast, in the present invention, the injection of the plastics melt can cause the mould parts to separate and the mould cavity to expand. The light pressure used to close the mould initially is intended primarily to exclude gas from the mould cavity. This is to avoid gas pockets being trapped in the cavity. As the melt is introduced into the mould, the cavity can expand as necessary so that the melt flows relatively freely to occupy part of the volume of the cavity. Once the predetermined quantity of molten plastics material has been introduced into the mould, the parts of the mould are brought together under high pressure to reduce the mould cavity to its final volume and force the melt to flow into all parts of the cavity.
It follows from the above explanation that the quantity of the plastics material needs to be predetermined because the injection cannot simply continue until the cavity is totally filled and the back pressure prevents further injection of the plastics material into the mould. In the prior art, on the other hand, injection is stopped by back pressure, at which time the plastics material may already occupy some ninety percent of the cavity. The final reduction in volume of the cavity is used only to force the partly solidified plastics material to flow into the last ten percent of the mould cavity.
In a typical embodiment of the present invention, the relative displacement of the mould parts under pressure is in excess of ten times the final mould thickness and may be as great as two hundred times the final moulding thickness. This is to be contrasted with a corresponding movement of some twice the final wall thickness, that is typically used in injection compression moulding.
A further important difference between the invention and conventional injection compression moulding resides in the speed of closing the mould and the rate of pressure increase within the mould cavity during the closing process. In the present invention, the mould is closed and maximum pressure is reached within the cavity within a period of less than 0.5 seconds and preferably less than 0.3 seconds. By contrast, in injection compression moulding, after the plastics material has been injected under pressure to fill a major part of the mould and cavity, the pressure is ramped up progressively to flow the plastics material to fill the remainder of the mould.
The most important difference however between the invention and earlier proposals resides in the application of a light pressure to close the mould while injection is taking place and a higher pressure to compress the injected plastics material after completion of the injection phase.
The relevance of this difference will be described by reference to the moulding of drinking cups, this being an example of an article to which the invention particularly lends itself. When forming a drinking cup, the plastics material is injected into the base of the cup and the application of high pressure to close the mould after completion of the injection step forces the plastics material to flow upwards from the base to form the side walls of the cup and any lip surrounding the mouth.
In such an application, it is essential to avoid gas being trapped in the bottom corners of the mould cavity at the junction between the base and the side walls. Compression of such gas raises its temperature and causes unacceptable burn marks in the finished products.
If the plastics material were injected into a fully open cavity, it would form a small sphere which would be flattened when the mould parts are brought together at high speed and would trap gas in the corners of the moulds. In the present invention, this is avoided because the injected plastics material does not form a sphere in the first place. Instead, the mould is closed or at least nearly closed under light pressure and the injected plastics material flows as a radially expanding disc which forces the mould parts apart if necessary until it totally fills the base of the cup. When the pressure is increased to force the injected plastics material up the side walls of the cup, there will be no gas trapped in the corners to cause burn marks.
It is important that the cavity volume should be fully contained at all stages after the commencement of injection of the plastics melt. Although the volume of the cavity must be variable, the melt must not be allowed to escape from the cavity.
To permit implementation of the method of the invention, the invention also provides in accordance with a second aspect, a mould for mounting between the platens of an injection moulding machine for injection compression moulding of a thin walled article, the mould comprising a first mould part, a core and a rim closure part surrounding the core, the three parts together defining a mould cavity and being movable relative to one another in the direction of opening and closing of the mould, wherein the rim closure part sealingly engages the first mould part and is in sealing sliding engagement with the core, movement of the core enabling the volume of the cavity to be varied while plastics material injected into the mould cavity remains fully contained.
In a further aspect, the invention provides a method of injection compression moulding a thin walled article using a mould as set forth above, which comprises the steps of
moving the three parts of the mould towards their closed position in which they define a sealed cavity of variable volume,
injecting a predetermined dose of plastics material into the cavity, the injection terminating after the cavity has been fully enclosed, the core being allowed to move away from the first mould part during the injection of plastics material into the cavity while offering a resistance lower than the force exerted by the material injected under pressure thereby enabling the cavity volume to increase to accommodate the volume of the injected dose, and
forcing the core back into the first mould part to compress the injected plastics material and cause the material to flow into the parts of the mould cavity that define the thin walled sections of the finished article.
The mould is preferably provided with means for aligning the rim closure part relative to the first mould part in order to achieve accurate alignment of the movable core in relation to the remainder of the mould cavity.
A spring and/or a damper is preferably provided to resist movement of the core away from the first mould part.
In the preferred embodiment of the invention, a lost motion coupling is arranged in the line of force between the machine platen and the core. When the platen first moves in the direction to close the cavity, the core moves with it and the rim closure part closes the cavity. As plastics material is injected into the cavity, the presence of lost motion allows the core to move back towards the moving machine platen so that the cavity expands or at least the cavity does not continue to contract at the rate of movement of the platen. The lost motion is fully taken up by the time that the injection is completed and thereafter the core again moves with the platen to compress the cavity to its final minimum volume.
One can envisage an implementation of the invention using a moulding machine with a hydraulic lock (i.e. one that uses only hydraulic actuators to apply all the necessary pressure to the mould parts). Using such an approach, it is more difficult to achieve the necessary speed of closure of the mould cavity as well as the rapid rise in pressure that is needed as the mould parts approach their closed position. This may therefore require the use of purpose built machines. In this respect, it should be noted that the plastics material is cooled and sets as it makes contact with the mould surfaces and it vital for this reason that the flowing of the plastics material by the closing of the mould parts should be completed as quickly as possible. An injection moulding machine with a toggle closure mechanism has however been found to be well adapted to provide the substantial force require to effect the final closure.
Since the pressure in the plastics material on final closure is substantial, the injection gate through which the plastics material is injected into the mould is preferably closed by a valve prior to final mould closure. This is to avoid expulsion of material from the tool during final closure.
Preferably, the closure of the mould under high pressure involves reduction of the mould part gap over a substantial portion of the surface area of the finish formed article, whereby movement of the plastics material through a thin mould part, which might otherwise be regarded as too thin, occurs only during the last part of the final closure.
In order to introduce an accurately predetermined quantity of molten plastics material, it is preferred to provide a hot-runner system that comprises a manifold incorporating a dosing cylinder for each of the cavities, each dosing cylinder being connected to a common pressurised supply of molten plastics material by way of a respective valve and being connected to the associated cavity by way of a gate valve.
Each dosing cylinder comprises a variable volume chamber bounded by a piston which acts to store the required dose of plastics material for its associated cavity. When the plastics material is injected from the pressure source into the dosing cylinders, it will first flow to the dosing cylinder offering the least resistance but when this pot is full the plastics material will meet resistance and will be diverted to another of the dosing cylinders until all the dosing cylinders are full. An adjustable stop may be provided for each of the pistons of the dosing cylinders to allow fine adjustment of the quantity of plastics material delivered to each cavity. When the gate valves are opened and the pistons of the dosing cylinders are moved in a direction to reduce the volume of the working chambers, the stored plastics material is forced past the gate valve into the cavity and is prevented from moving in the opposite direction.